The present invention relates to image processing apparatuses, image processing systems, image processing programs and storage media for image processing programs.
Digital image processing apparatuses of these days are required to process high-quality image data characterized by high resolution and large number of gradations. Because such high-quality image data includes a large amount of information therein, the image processing apparatus used for processing such high quality image data is subjected to severe processing load. For example, a black-and-white image of 256 gradations has the amount of information of eight times as large as that of a binary image (black-and-white image). Accordingly, the image processing apparatus used for processing such an image is required to have a memory capacity of at least eight times as large as the case of processing binary black-and-white image merely for the purpose of storing the image data, and thus, the cost of the apparatus increases inevitably.
Because of this, it has been practiced to reduce the necessary memory capacity by encoding the image to be processed.
For the encoding method capable of reducing the amount of data efficiently, the use of JPEG is recommended by ISO (INTERNATIONAL ORGANIZATION FOR STANDARDIZATION) and ITU-T (INTERNATIONAL TELECOMMUNICATION UNION-TELECOMMUNICATION STANDARDIZATION SECTOR) as the standard process for compressing high quality image data and for encoding multiple gradation images including color images.
JPEG includes a basic process that uses DCT (discrete cosine transformation) and an optional process that uses DPCM (differential pulse code modulation). The DCT process encodes the original image information to frequency information by using discrete cosine transform. This process utilizes visual characteristics of human beings and achieves the desired reduction of the amount of information of the original picture in such a manner that there is caused no degradation in the picture quality. This encoding process is a so-called irreversible encoding process.
On the other hand, the latter method that uses DPCM predicts the level of the picture element of attention from the surroundings picture elements and encodes the error of the prediction. According to this process, it is possible to encode the original picture without degrading the quantity of the information of the original picture. Thus, this is a reversible encoding method. Reference should be made to Japanese Patent 3,105,906.
In order to encode image data while maintaining high picture quality, the use of a DCT process having high efficiency is suited. However, from the viewpoint of preservation of the original information, the use of a reversible DPCM process is more preferable than the irreversible DCT method.
Ideally, it is desirable to use a reversible encoding process capable of conducting high efficiency compression simultaneously. However, the state of art reversible encoding process that uses the DPCM process cannot provide satisfactory efficiency of data compression. Because of this, the DCT process is used commonly in personal computers, and the like, for compressing multi-level images having a relatively large number of gradations.
However, in the case of the DCT process, there is a tendency that the picture quality is deteriorated extremely as a result of occurrence of block distortion pertinent to this process or the formation of mosquito noise in the outline part of the images when the compressibility is increased. This tendency appears conspicuous particularly in character images and raises a serious problem in the picture quality.
Further, while the JPEG process may be an optimum process for reducing the memory capacity necessary for holding images in an image processing apparatus, it is not an optimum process for the purpose of editing or processing of images used with digital copying machines or multi-functional machines (MFP), and the like. This is because the JPEG process cannot specify a position in the image in the encoded state. In other words, the JPEG process cannot decode a particular part of the image specified arbitrarily.
When conducting editing of image data in the JPEG process, it has been necessary to decode all the image data and conduct the desired editing process to the decoded image data. Thereby, there is a need for conducting the process of encoding the edited image again. During such a process, there arises a problem that a very large memory is used for holding the decoded image. For example, it should be noted that an RGB color image of A4 size, 600 dpi resolution, needs the memory size of about 10 Mbytes.
As a method capable of solving this problem of memory usage at the time of edit processing, it is conceivable to use an encoding process of fixed length. Reference should be made to Japanese Laid Open Patent Application 11-144052, for example.
It should be noted that the encoding technology of images is divided largely into the variable length process and the fixed length process according to the code word length after encoding.
Here, the former has the advantageous feature of good encoding efficiency and enabling reversible encoding, while in the case of the latter process, it is possible to know the location in the image after encoding in the encoded state. This means that it is possible to reproduce the image of an optional part arbitrarily. In other words, this means that edit processing, and the like, becomes possible in the encoded state.
On the other hand, the fixed length encoding process has drawbacks in that encoding efficiency is generally poor as compared with the variable length encoding process and that reversible encoding is difficult.
On the other hand, the encoding process called JPEG 2000 attracts attention these days as the process capable of solving the drawback of the abovementioned JPEG encoding process.
JPEG 2000 is a transformation encoding process that uses a wavelet transformation, and it is predicted that JPEG 2000 will replace JPEG in the field of still images including color images from now on.
JPEG 2000 has various novel and practical functions, in addition to the feature of reducing the problem of picture quality deterioration in the low bit rate, which is pertinent to JPEG. In such a novel function, there is a process called tile processing. It should be noted that this tile processing is a technology that divides an image into small regions. The encoding is conducted in these small regions independently. As a result of this function, it becomes possible to specify a particular desired region of an image in the encoded state and it becomes possible to conduct edit processing in the encoded state.
On the other hand, while JPEG 2000 encoding process has such various desirable features, there is a problem that the processing becomes inevitably complex in order to realize such various functions and simultaneously a high performance process.
In the case of JPEG2000, for example, a processing time of about 4-5 times is needed when processing by software as compared to the case of using the JPEG encoding process, and there arises a serious problem of operability of the user in the applications for editing.
In more detail, there are many cases, when editing images, in which it is desirable to apply direct processing to the images subjected to editing, while such a method requires a large workspace when handling a large-size image. In the case of conducting editing by utilizing a personal computer, for example, a large memory size becomes necessary for storing the image.
Especially, there are cases these days to connect an image formation apparatus such as a digital copying machine or a multifunctional machine (MFP) to a personal computer via a network for conducting an editing operation not capable of being performed through an operational panel of the image formation apparatus, such as erasing of traces of stapler or punch holes in a manuscript. In such editing, there frequently occurs a process of incorporating the image data of a scanned manuscript into a personal computer, applying a desired editing to the image data with the personal computer, and returning the image data thus applied with the desired edit processing to the image formation device and printing out the edited image. During such editing, it should be noted that there frequently occurs the situation in which the memory area or data storage device of the personal computer is occupied by the large image data.
In order to solve this problem and to save the memory area of the personal computer, it is conceivable to place only the encoded image in the memory area. However, in the case of editing images of high picture quality, it is necessary to use a reversible encoding process characterized by a large data quantity, and it is generally inevitable that the processing speed becomes slow associated when processing large-size data. Also, in the case the encoding process allows random access to the encoded data, it becomes necessary to conduct a decoding process on the entire encoded image data each time, and it is a possibility that the processing speed becomes slower than storing the images directly.